1. Field of the Invention
This invention relates to a method for purification of 2-methyleneglutaronitrile (hereinafter sometimes referred to as MGN).
MGN is an important chemical to be used as an intermediate for pharmaceuticals, agricultural chemicals and various polymers. Accordingly, various methods for preparation thereof have heretofore been developed.
Of these methods for preparation, a method of dimerizing acrylonitrile in the presence of a catalyst composed of a metal halide and a trialkylamine as disclosed in U.S. Pat. No. 3,733,351 is an excellent method.
A distillation method may be thought to be suitable for recovering the objective MGN from the reaction product obtained by the above mentioned method. It has been found, however, that there exist, unexpectedly, various technical problems in the recovery and purification by distillation of MGN in commercial production. For example, the reaction product contains the metal halide used as the catalyst, and the metal halide accelerates polymerization of MGN when the reaction product is distilled at a temperature of about 100.degree. C. or higher, thereby lowering the yield of MGN. Moreover the polymerization reaction progresses explosively and is dangerous. Further, in the above-mentioned reaction of acrylonitrile, a portion of the trialkylamine used as the catalyst and the metal halide form an adduct, which is fixed in the reaction product. The adduct is gradually decomposed in the course of the distillation and the trialkylamine thus yielded goes as an impurity into the distillate MGN. No prior art references dealing with commercial purification of MGN have been found.
2. Possible Solution of the Problems
We had found that the problems encountered in the case of recovering MGN from such crude MGN can be solved by pretreating the crude MGN with an alkali. According to this method, the metal halide is converted into the corresponding hydroxide and is separated out in the crude MGN liquid to lose its catalytic action. On the other hand, the adduct of the trialkylamine and the metal halide is decomposed, and the trialkylamine thus yielded is also driven out of the alkaline crude MGN liquid. Although some of the trialkylamine may still remain in the crude MGN, it can be trapped as a low-boiling substance in an initial distillate fraction in the course of the distillation. Thus, the above mentioned problems observed in the direct distillation of crude MGN can be solved.
It has been found, however, that there still remain some problems in this method. That is, the metal hydroxide formed by the alkali treatment is separated out as very fine particles, and the particles are difficult to remove. Moreover, in the distillation of the alkali-treated MGN, MGN is apt to polymerize because of a slight amount of the alkali present.